Which memory synchronization primitive is used to protect a critical section so that only one thread can enter at a time?

• A. Condition variable
• B. Mutex
• C. Spinlock
• D. Semaphore

Answer: (B)

Mutual exclusion is the goal: ensure that only one thread can execute a critical section at a time. A mutex provides this by requiring a thread to acquire the lock before entering the protected region; if another thread tries to enter while it's held, it blocks until the lock is released. This blocking behavior saves CPU time and establishes a clear ownership and memory visibility order: changes inside the critical section become visible after unlocking. While condition variables are used with a mutex to wait for conditions, they don’t by themselves protect a section of code. Semaphores control access with a count and can allow multiple concurrent entries unless the count is 1, and spinlocks busy-wait, which can waste CPU time for longer waits. For protecting a critical section so only one thread enters at a time in typical scenarios, a mutex is the appropriate primitive.